1. Field of the Invention
This invention relates to aircraft instruments, and more particularly to such instruments that provide outputs indicative of wind shear conditions. Wind shear may be defined as the condition that exists when the velocity and/or direction of the wind is significantly changing with altitude, horizontal position or time, or any combination thereof. Wind shear can pose a particularly hazardous situation when an aircraft is making a landing approach to an airport and is descending through an air mass that exhibits significant wind shear. Such wind shear can cause the airspeed experienced by the descending aircraft to increase and decrease significantly within short periods of time. An increase in airspeed may mislead the pilot into reducing aircraft power, and a decrease in airspeed can reduce the airspeed of the aircraft below the speed at which safe control of the aircraft may be maintained.
Wind shear also, may include local updrafts and downdrafts as well as changes in the horizontal component of the velocity of the wind. Thus, wind shear may cause changes not only to the airspeed of the aircraft but may also cause vertical accelerations of the aircraft thus causing it to depart from the flight path it otherwise would follow in still air. Such vertical departures from flight path are characterized herein by the downdraft drift angle.
A warning to the pilot of the existence of a dangerous level of such wind shear conditions can alert the pilot to incipient changes in the airspeed and to deviations from the flight path so as to enable the pilot to take immediate corrective action.
2. Description of the Prior Art
U.S. Pat. No. 4,012,713 describes a system for providing an output signal indicative of a wind shear condition. U.S. Pat. No. 4,079,905 ("905") describes an improved system which additionally senses the downdraft drift angle of the aircraft i.e. the departure from the normal approach flight path. The "905" device combines a signal representing the downdraft drift angle with a signal representing the rate of change of the instantaneous airspeed of the aircraft to provide a composite signal which is used to provide a warning to the pilot of incipient dangerous flight conditions.
As a matter of good piloting practice, whenever significant wind shear conditions are expected, the pilot normally selects an approach airspeed that is somewhat greater than the airspeed that would be used in the absence of such anticipated wind shear conditions. The additional approach airspeed operates to provide an increased margin of safety in circumstances where wind shear is significant. As a consequence, in order for a wind shear warning system to provide a warning of incipient unsafe flight conditions, while at the same time not giving an excessive number of warnings when such unsafe conditions are not imminent, the warning system should include a mechanism for adjusting the warning threshold in accord with the pilot selected excess of the approach airspeed over the "normal" approach airspeed.